1. Field of the Invention
The invention relates to the field of wireless local area networks and proposes systems and methods for scheduling traffic over a local area network. The invention is particularly relevant to wireless local area networks where traffic is categorized and where priorities are given to traffic categories based on respective associated timing constraints. The invention is further relevant to wireless systems operating according to the IEEE 802.11e specification.
2. Description of the Related Art
Personal computers (PC) and traditional consumer electronic (CE) devices such as set-top boxes, television sets, sound systems and the like are slowly converging and both PCs and CE devices now offer functionalities that tend to satisfy customers' needs in similar ways. Wireless technologies provide this link between the two areas of expertise and CE manufacturers are working on bringing the PC content to the living room and/or, vice versa, on bringing living room content such as TV programs to the user's PC. A major concern with wireless communication is however to ensure data delivery over an unreliable and crowded environment. In addition, some traditional problems, e.g. delays, interferences, losses, encountered over wireless mediums when transmitting non-multimedia data such as text files or emails are only exacerbated when dealing with multimedia content. Indeed, multimedia content does not tolerate packet losses or delays and IEEE 802.11e seeks to solve this issue by defining quality of service for data transmission over WLANs. In order to do that, IEEE 802.11e introduces traffic prioritizing, traffic negotiation and traffic scheduling.
The basic architecture in a wireless local area network as defined in IEEE 802.11 is a basic service set (BSS). A BSS is a set of stations that can communicate with one another directly as it is done in independent basic service sets, or via an access point, as done in infrastructure basic service sets. Stations can transfer data following one of several access mechanisms. A first access mechanism is known as carrier sense multiple access with collision avoidance (CSMA/CA) according to which a station senses the wireless medium before beginning a transmission. The station can either use a physical carrier sensing mechanism or, preferably, the station can use a virtual carrier sensing mechanism as provided in the IEEE 802.11 specification with the network allocation vector (NAV). Such access mechanism is also referred to as the distributed coordination function (DCF), which is a contention based channel access that all stations use when they compete for the medium. A second access mechanism is a centrally controlled one that uses a poll and response protocol to avoid collision and contention. This access mechanism is also called the Point Coordination Function and is managed by a Point Coordinator (PC) usually located in an access point (AP). Stations transmit requests to the PC that logs the requests in a polling list and polls the stations one after another for traffic while also delivering traffic to them. The PCF is the preferred access mechanism when time-sensitive multimedia content is transmitted over a crowded environment. The PCF may be used together with the distributed coordination function (DCF). To this end, the Point Coordinator has a timing advantage over other stations in the BSS in taking over the medium and, once the Point Coordinator has gained access to the medium, it may then negotiate and control the transmission of time-sensitive data, e.g. content streaming, from one station to another.
The IEEE 802.11e lays down the basis for traffic categorizing and traffic controls and how traffic is actually scheduled is left to the manufacturer's choice. There is therefore a need for a reliable and efficient scheduling scheme.